Justin Timberlake cheated on Jessica Biel with actress Olivia Munn during a three-day fling this past September, reports the new Us Weekly (on newsstands now).

Timberlake, 29, and Munn, 30, a Daily Show correspondent who stars in the upcoming NBC series Perfect Couples, first met at a Sept. 26 MySpace event.

After exchanging numbers, he "started chasing her immediately," says an insider.

Munn resisted his advances, telling him it was a no-go if he was still with Biel, 28, the source tells Us Weekly. But Timberlake "has been telling people it’s over with Jessica, even though "the reality is he's just doing it behind her back."

Believing he had left Biel and that her relationship with Timberlake might develop into something serious, Munn took him to her hotel, the Gansevoort Park Avenue NYC, Sept. 27 and 28, where "they were openly affectionate," the insider adds.

The insider tells Us the two "had amazing sex" that night.

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Timberlake's rep strongly denies the allegations; Munn’s rep does not comment on her personal life.

For more - including what drove a sad and "used" Munn to end the fling and why Biel stays with Timberlake -- pick up the new Us Weekly today!